The invention relates in general to thin-film solar cells using a non-single-crystal thin-film. More specifically, the invention provides a non-single-crystal solar cell that is especially efficient in post-irradiation,
When compared to monocrystal silicon solar cells, PIN thin-film solar cells, having at least one PIN junction, namely, a lamination of a p-type semiconductor layer of non-single-crystal thin-film (hereafter referred to as xe2x80x9cp-layerxe2x80x9d) , a substantially intrinsic i-type semiconductor layer (referred to below as xe2x80x9ci-layerxe2x80x9d), and an n-type semiconductor layer (referred to below as xe2x80x9cn-layerxe2x80x9d), and especially PIN thin-film solar cells, using non-single-crystal thin-films such as silicon-derived amorphous silicon (referred to below as xe2x80x9ca-Sixe2x80x9d), microcrystalline silicon (referred to below as xe2x80x9cxcexcc-Sixe2x80x9d), and polycrystalline silicon, can be made large in area, at low temperatures, and at low cost, and as such, are promising for use as a large-area thin-film solar cell to generate electric power.
xe2x80x9cThe substantially intrinsic i-layerxe2x80x9d described in this specification is not limited to undoped semiconductor layer. Amorphous semiconductors, even if intentionally undoped, are sometimes automatically doped by diffusion of residual gas or impurity emitted from the inner surface of a chamber wall during deposition, and thus Fermi energy may be shifted from the center of the bandgap. A semiconductor layer compensated such that the Fermi energy is located at the center of the bandgap by microdoping p or n-type impurity also includes xe2x80x9csubstantially intrinsic i-layerxe2x80x9d.
Examples have been reported where wide-gap a-Si alloy and xcexcc-Si were applied as the material for the player, the window layer of the PIN-type solar cell, for the purpose of increasing the efficiency of the non-single-crystal solar cell. Amorphous silicon carbide (hereafter referred to as xe2x80x9ca-SiCxe2x80x9d), amorphous silicon oxide (hereafter referred to as xe2x80x9ca-SiOxe2x80x9d), and amorphous silicon nitride (hereafter referred to as a-SiN), among others, were brought together to form the a-Si alloy. It has been shown that using these materials in the p-layer results in increased short circuit current density (hereafter referred to as xe2x80x9cJscxe2x80x9d) due to a reduction in light absorption loss, and an increase in open-circuit voltage (hereafter referred to as xe2x80x9cVocxe2x80x9d) from increased diffusion potential.
It is known, however, that non-single-crystal solar cells, and a-Si solar cells in particular, experience reduced efficiency with illumination. As to the special features of the initial period, following irradiation there is, on the whole, a reduction in Voc, Jsc, and the curved factor (hereafter referred to as FF), respectively, and a decline in efficiency (hereafter referred to as Eff).
Until now, p-layer conditions have mostly been optimized with regard to the peculiar characteristics of the initial period, and what was optimized for the initial period was thought to be equally optimal for post-irradiation, but not much consideration has really ever been given to the peculiar characteristics of post-irradiation.
In contrast, there has recently been reported instances whereby application to the i-layer of an a-Si film made with hydrogen-diluted silane, resulted in an increase in post-irradiation Voc. For example, Siamchai et al., who used the photo-CVD method, report that when you raise the hydrogen dilution levels for the i-layer in an a-Si solar cell with no p-i interface layer, post-irradiation Voc increases against the initial period, and that the cause of this is attributable to a disparity in defect energy within the i-layer due to hydrogen dilution [Pavan Siamchai and Makoto Konagai, Proc. IEEE 25th Photovoltaic Specialists Conference (1996) pp. 1093; Pavan Siamchai and Makoto Konogai, Appl. Phys. Lett., Vol.67 (1995) pp.3468]. The post-irradiation Eff in a-Si solar cells with increased Voc from illumination, however, did not exceed the Eff in a-Si solar cells which includes the p-i interface layer and exhibits ordinary degradation under illumination. Furthermore, they are of the opinion that increases in Voc by irradiation depends on the conditions under which the i-layer is prepared.
Meanwhile, Isomura et al. report that when they used the plasma CVD method and diluted silane with hydrogen by ten times or more, they achieved an increase in post-irradiation Voc, and believe the reason has nothing to do with either the p-layer or the p-i interface layer [Masao Isomura, Hiroshi Yamamoto, Michio Kondo and Akihisa Matsuda, Proc. 2nd World Conference on Photovoltaic Energy Conversion, to be published]. However, even in this case, whatever increased Voc as a result of irradiation did not lead to a rise in Eff. This report also states that the increases in Voc by irradiation depend upon the conditions under which the i-layer is prepared.
Furthermore, Xi et al. report raising Voc through the optimization of the a-SiC p-layer comprised of large quantities of carbon (hereafter referred to as xe2x80x9cCxe2x80x9d), and p-i interface layer, and that whenever Voc was low in the initial period, post-irradiation Voc increased [Jinping Xi, Tongyu Liu, Vincent Iafelice, Martin Nugent, Kevi Si, Joe del Cueto, Malathi Ghosh, Frank Kampas, Proc. 23rd IEEE Photovoltaic Specialists Conference (1993) pp.821]. However, when post-irradiation Voc increased, no increases in Eff were exhibited. Further, nothing regarding any link between the conditions of p-layer or p-i interface layer preparation and the increases in post-irradiation Voc was reported in any concrete fashion.
An increase Eff has been sought using a wide gap non-single-crystal alloy and xcexcc-Si in the p-layer of our non-single-crystal PIN solar cell,. However, Eff needs to increase to even a greater extent before it can be put to real use.
Moreover, insofar as the conditions for the p-layer were concerned, it had been believed that the elements which had been optimized in accordance with the initial period""s peculiar characteristics would also be optimal in the post-irradiation period. The problem, however, is that these conditions were not necessarily optimal given the peculiar characteristics of post-irradiation. For example, as was mentioned above, Voc increased in post-irradiation because of the manner in which the i-layer was prepared, but without resulting in an increased post-irradiation Eff.
In light of this problem, it is an object of the present invention to provide non-single-crystal solar cell that is especially efficient in post-irradiation, and doing so by focusing on the peculiar characteristics of post-irradiation.
In order to resolve the problem discussed above, the inventors performed their own experiments by taking a p-layer, for example, whose Voc and other elements were optimized in the initial period""s peculiar characteristics, and observing closely what was not optimal about it in post-irradiation. By paying particular attention to the specific characteristics of post-irradiation, particularly with regard to efficiency, conditions were established that would maximize its efficiency. More precisely, taking non-single-crystal solar cell having at least one PIN junction, formed by laminating together of a p-type semiconductor layer made from non-single-crystal thin-film, a substantially intrinsic i-type semiconductor layer and an n-type semiconductor layer, things like film thickness and acceptor impurity levels of the above-mentioned p-type semiconductor layer were adjusted in an effort to achieve 0.85-0.99 times the maximum pre-irradiation open-circuit voltage value in this non-single-crystal solar cell before performing irradiation.
As described in greater detail below, experiments were conducted to change the material, film thickness and impurity levels in the p-type semiconductor layer. In each instance, the results endeavored to be achieved were the same: 0.85-0.99 times the pre-irradiation open-circuit voltage. In this case, it is possible to achieve a higher efficiency rate after the irradiation than in the case where the pre-irradiation open-circuit voltage is the maximum value.
The results will now be described in greater detail. Initial period Voc increased along with increased film thickness (Dp), and achieved its maximum at a certain thickness. The term xe2x80x9cDpsatxe2x80x9d will be used to identify the thickness at which initial period Voc reached its highest level. However, as Dp gets thicker, the p-layer light absorption loss increases and Jsc decreases. Hence, the efficiency rate in the initial period is at its highest somewhere around Dp=Dpsat.
Meanwhile, it was observed that Voc increases in the lab when irradiation on a non-single-crystal solar cell whose Dp less than Dpsat was performed. Next, the film thickness of Dpsat-2, at which post-irradiation Voc reached its highest point, was thinner than Dpsat. Due to the thinness of the Dpsat-2 film, the cells within its vicinity experienced a decrease in light absorption loss and an increase in Jsc. The post-irradiation efficiency rate of Dpsat-2 is, thus, higher than in Dpsat cells. When attempts are made to make the Dpsat-2 film even thinner, irradiation results in increased Voc, but unfortunately, Voc""s absolute value decreases. However, if balanced with the increased Jsc from a thin Dp, up to certain thicknesses the efficiency rate is higher than in Dpsat cells.
The parameters in which efficiency in Dpsat cells becomes higher were as follows: when controlling the p-layer thickness and impurity levels in an effort to attain 0.85-0.99 times the maximum value in initial period Voc, an increase in post-irradiation efficiency was consistently achieved. If Voc is lower than 0.85 times the maximum value of the initial period Voc, the post-irradiation efficiency is lowered.
The mechanism by which irradiation results in Voc increases is not presently understood, but it has been concluded that this increase in Voc is not something that occurs because of increases in the photocurrent, but because of decreases in the dark current.
Around the time of irradiation, the dark current voltage shifts to a high voltage, and it is believed that this results in changes in the distribution potentials of the transparent electrodes, the p-layer, the p-i interface, and the i-layer. Further, around the time of irradiation, on the semi-logarithmic plot the tendency of the dark current after irradiation is to get smaller, and the n-value of the diode is to get bigger, while the proportion of the recombination current vis-a-vis the diffusion current increases. Meanwhile, it is believed that the reason why the shift in the voltage arising from dark current appears as a Voc increase is because the absolute value of dark current, occurring in bias currents whose voltage is lower than Voc, is smaller in comparison to that of photocurrent, and because it is not very efficient as a leakage current.
Furthermore, when heat annealing for several hours at 100xc2x0 C.-150xc2x0 C. is performed, the increases in irradiation induced Voc return to their initial period values. It is, therefore, not a process whereby the diffusing impurities and the like are irreversible. It can be considered that the increase of Voc resulting from the irradiation relates to a reversible process such as the generation of flows that arises from the irradiation and the elimination of the flows through heat annealing.
The p-type semiconductor layer is comprised of an amorphous silicon alloy made from a-Si or a-SiO, a-SiC, a-SiN, xcexcc-Si or microcrystalline silicon oxide, microcrystalline silicon carbide, microcrystalline silicon nitride; or polycrystalline silicon.
Although experiments were performed on p-type semiconductor layers made of a-SiO and xcexcc-Si, it is believed that similar results would be obtained using the other materials listed above and wide-gap semiconductors.
Below is documented the results obtained when a-SiO is used for the p-type semiconductor layer, and set the thickness of said p-type semiconductor layer at 0.25-0.83 times the thickness of a p-type semiconductor layer whose pre-irradiation open-circuit voltage is the greatest, specifically when the thickness is set at 3-10 nm.
Below is documented the results obtained following irradiation when xcexcc-Si is used for the p-type semiconductor layer, and the thickness of said p-type semiconductor layer is set at 0.08-0.83 times the thickness of a p-type semiconductor layer whose pre-irradiation open-circuit voltage is the greatest, specifically when the thickness is set at 5-50 nm.
It is also advisable to have a p-i interface layer made from non-single-crystal thin-film between the layers of the i-type semiconductor and the p-type semiconductor.
Whether there was or was not a p-i interface layer had somewhat of an effect on the absolute value of Voc or efficiency xe2x80x9cEffxe2x80x9d. But in either case, increases in Voc following irradiation were observed when p-type semiconductor layers were in the thin range and obtained similar results.
The experiments made it clear that if the irradiation is performed for 10 hours or more at a light intensity of 1 SUN, or on conditions that (a light intensity [SUN])2xc3x97(hour[h])  greater than 10, the characteristics are stabilized.